30 TH I NTERNATIONAL C OSMIC R AY C ONFERENCE

Search for point sources of gamma rays using GRAPES-3 experiment
A. O SHIMA2 , S.K. GUPTA1 , K. HAYASHI2 , Y. H AYASHI2 , N. I TO2 , A. I YER1 , P. JAGADEESAN1 ,A. JAIN1 , S. K ARTHIKEYAN1 , S. K AWAKAMI2 , H. KOJIMA3 , T. M ATSUYAMA2 , M. M INAMINO2 ,P.K. MOHANTY1 , S.D. M ORRIS1 , P.K. NAYAK1 , T. N ONAKA2 , S. O GIO2 , T. O KUDA2 , B.S. R AO1 ,K.C. RAVINDRAN1 , M. S ASANO2 , N. S HIMIZU2 , K. SIVAPRASAD1 , H. TANAKA1 , S.C. T ONWAR1 ,T. YOSHIKOSHI2
1Tata Institute of Fundamental Research
2Osaka City University
3Nagoya Women’s University
oshima@alpha.sci.osaka-cu.ac.jp
Abstract: The GRAPES-3 experiment observes extensive air showers using a high-density array ofscintillation detectors and a large area tracking muon detector. The showers due to charged cosmic rays
can be identified by measuring their muon content using tracking muon detector. We have been able todetermine the angular resolution of the array using the shadow of Sun and Moon. Here we report on thestatus and some results of the search for gamma rays from the ‘standard candle’ CRAB nebula using theGRAPES-3 data.

Introduction
GRAPES-3 experiment is located at Ooty (E76.7◦ ,N11.4◦ , 2200m asl), India. The GRAPES-3 air
shower array with a large field of view and long
duty cycle can observe the northern sky by measuring the secondery particle distribution of extensive air showers originated from the primary cosmic rays. The array consists of about 300 scintillation detectors each having an area of 1m2 with
8 m separation from each other and 16 large area
muon detectors having a total area of 560m2 . Triggar rate of the array during the period of this analysis is about 13Hz. To identify the showers to
be charged particle induced or gamma induced,
we check the track of the muons accompanying
the showers recorded at muon detectors with thereconstructed air shower direction. Detail of themuon detectors is mentioned in the next section.
The search for gamma ray point sources is oneof the most important goal of GRAPES-3 experiment. Because primary gamma rays produce extensive air showers in the atmosphere which shouldhave very few number of muons, we can classify
showers according to their muon content and sup-Figure 1: A muon station has four muon detector modules each consisting of 232 proportionalcounters. There are four muon stations inside
the air shower array with a clustering arrangement(Figure2).
press the huge background level of hadron-induced
showers by using large area tracking muon detectors. The most famous gamma ray point source is
the Crab Nebula which has a steady emission ofTeV gamma rays. We have carried out a search for
gamma ray point sources especially for Crab Nebula using the data recorded during 2000-2003. Theresults of the observation on the Crab Nebula andthe effect of hadron cut by using the muon detector
will be presented here.

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Muon detector
GRAPES-3 large area (560m2 ) tracking muon detector which allows us to distinguish showers initiated by primary gamma rays from that of charged
cosmic rays through a cut on the hadron content.
It is very important feature of our array for gamma
ray point source search. A muon detector modulewith a sensitive area of 35m2 consists of 232 proportional counters arranged in 4 layers, each having 58 counters (Figure1). Each counter has a crosssectional area of 10cm × 10cm and 6m long. Two
successive layers of proportional counters are separated by 15cm thick concrete. Because alternatelayers are placed in orthogonal directions, track ofmuons can be measured with an accuracy of about6◦ .

Event selection
To select properly reconstructed showers we applied some quality cuts. Ratio of number of particles detected in the outer 2 rings of scintillation detecter array to a total number of particles should beless than 0.35 for the purpose that the shower core
could be estimated properly. The estimated shower
core must be inside the hexagonal area masked
with red color in the Figure2. The zenith angleof reconstructed air shower direction is required tobe less than 40◦ . The data which seems to havenoisy counters was removed from analysis. And
the data set with an amplitude of azimuthal angle
distribution of more than 1.5% was not used in thisanalysis, because the angular resolution is not goodin that period. The data set in this analysis wasrecorded between 2000 March and 2003 May. Total number of events after the above quality cuts
becomes 8.9×105 . This data set was used for theCrab analysis.

Analysis and results
We already have information about the angularresolution of GRAPES-3 air shower array from
the Moon shadow analysis using equal solid angle
method in which the search area centered on theMoon position was divided into several bins of anannuli each having same solid angle[1]. BecauseFigure 2: GRAPES-3 air shower array in the periodof 2000-2003. Each small box indicates a scintillation detector and each gray box indicates a muon
detector module.

Figure 3: Variation of shower rate with opening
angle centered on the Moon. Each bin has same
solid angle. Angular resolution was calculated tobe 1.13◦ with the shower size of log10(Size) ≥ 3.2.

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all bins have the same solid angle, same numberof showers will come into each bin if there is noastrophysical object shadowing cosmic rays. The
angular resolution of array used during this analysis is 1.13◦ with the shower size of log10(Size)≥ 3.2. We show the deficit of shower rate due toshadowing by the Moon as a sample in Figure3.
In this analysis, we looked at the direction of CrabNebula using the equal solid angle method in thesame way as for the Moon shadow analysis. We set
the center of circular search window with a radiusof 4◦ on the position of Crab Nebula. Because wewant to know the effect of hadron cut using muon
detector also, the analysis was carried out in twosteps, first the data set was analyzed without applying muon cut, next the data was analyzed withthe muon cut. We also looked at another region tosee the background area at the same time. Background area was selected by shifting the positionof actual Crab in the right accension by ±8◦ withthe same search window.

Figure 4: Excess number of events from Crab direction without hadron cut applied. Significanceof the excess is about 1.0σ. The value of ”p1” is0.084.
The data was fitted by assuming the shape of theexcess part on the center of a search window to be
2D gausian like. There are two fitting parametersin each graph, ”p0” represents the level of background and ”p1” represents a ratio of the excess
amount to a ”p0”. The parameter ”p1” is a kind ofindicator of an excess. We used the value of ”p0”of the off-source data as the background, because
fluctuation of the off-source region is large. Therefore the significance derived in this analysis was

Figure 5: Two background regions were added together. Background regions were set at the place
distant from the Crab position by ±8◦ in the rightaccension. The value of ”p1” is 0.063.
slightly over estimated. Figure4 and Figure5 show
the on-source region and the off-source region ofthe data set without hadron cut respectively. The
value of parameters ”p1” in the both the data arevery small and almost same together. We can’t see
an enough excess in the Figure4.
We applied the hadron cut which selects only theshowers having no muon recorded in the muon detectors. But this hadron cut used in this analysis
is a very simple, because gamma induced showers
also have some hadronic content even thogh theprimary particle is a low energy gamma ray andhave a lateral distribution of muons which depends
on the shower size. For higher energy, the provability of gamma induced showers containing hadronic
content becomes large.
After applying hadron cut to all data, the total number of events reduced from 8.9×105 to 2.3×105 .
Figure6 and Figure7 show the on-source region
and the off-source region for the data set withhadron cut applied respectively. The parameter”p1” of the on-source data is 0.274 and of the offsource data is 0.065. The latter is almost same asthat of off-source data without hadron cut applied.
The ”p1” of the on source data with hadron cut is3.3 times larger than without hadron cut. This suggests that we were able to suppress the backgrounddue to charged primary particles effectively.

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Acknowledgements
We thank all members in OOTY for help in construction and operation of GRAPES-3 experiment.

References

Figure 6: Excess number of events from Crab withhadron cut applied. Assuming the backgroundlevel to be a half of Figure7, significance becomesto be about 3.2σ. The value of ”p1” is 0.274.

Figure 7: Two background regions were added together. The value of ”p1” is 0.065.

Summery
We analyzed the data from 2000 March to 2003
May to see Crab Nebula which is well known as’Standard Candle’ in the gamma ray astronomy.
We got an excess number of events from the CrabNebula by using tracking muon detectors to reject
the showers containing muons. Using muon detectors we can increase the sensitivity of GRAPES-3
air shower array to gamma induced showers. Inthis analysis we selected only muon zero showers
as representing gamma ray induced showers.

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In 30th International Cosmic Ray Conference,Merida,2007.
[2] A.Borione et al. Obsevation of the shadows ofthe Moon and Sun usinf 100 TeV cosmic rays.
Physical Review D, 49:1171–1177, 1994.
[3] Y. Hayashi et al. A large area muon tracking
detector for ultra-high energy cosmic ray astrophysics - the GRAPES-3 experiment. Nuclear Instruments and Methods in Physics Research A, 545:643–657, 2005.